Machines often include an electrical system for facilitating operation of the machine and/or increasing the comfort and satisfaction of an operator of the machine. Such electrical systems may include one or more power sources that produce electrical power, one or more power-consuming devices, and power-transmission systems for distributing power from the power sources to the power-consuming devices. The power sources of such electrical systems may include an electric generator drivingly connected to a mechanical power source, such as an engine. The power capacity of such an electric power source may depend upon the speed at which the mechanical power source drives the electric generator. Often, the mechanical power source of the power unit is operated at a substantially constant speed and drives the electric generator at a substantially constant speed, while an operator is allowed full control over the operating state of the power loads. Unfortunately, this approach may result in power deficits if the operator controls the power loads in such a manner that their aggregate power need exceed the power capacity of the power source with the mechanical power source driving the electric generator at that particular speed.
U.S. Pat. No. 6,777,822 (“the '822 patent”) discloses a method of operating an electrical system, including adjusting the speed at which an engine of an APU (auxiliary power unit) drives an electric generator of the APU dependant upon the power needs of the power loads of the electrical system. The '822 patent discloses an electrical system with a APU including a gas turbine engine that drives a permanent magnet generator to produce electricity. For a given operating speed of the gas turbine engine, the output voltage of the permanent magnet generator depends on the current draw of the power loads connected to the permanent magnet generator. As the current draw of the power loads increases, the output voltage of the permanent magnet generator decreases and vice versa.
The method disclosed by the '822 patent includes controlling the operating speed of the gas turbine engine to maintain the output voltage of the permanent magnet generator at a target level. If the current draw of the power loads increases and causes the output voltage of the permanent magnet generator to fall, the operating speed of the gas turbine engine is increased to increase the power output of the permanent magnet generator and bring the output voltage of the permanent magnet generator back toward the target level. Similarly, the operating speed of the gas turbine engine is decreased if the output voltage of the permanent magnet generator rises above the target level.
Although the control method of the '822 application adjusts the speed at which the gas turbine engine drives the permanent magnet generator dependant upon the power needs of the power loads, certain disadvantages persist. Adjusting the speed at which the gas turbine engine drives the permanent magnet generator, by itself, may provide a limited ability to effectively respond to changing conditions in which the electrical system operates. For example, in some circumstances, the power needs of the power loads may rise quickly enough to overwhelm the APU and prevent the gas turbine engine from achieving a speed increase to accommodate the increased power needs.
The control methods of the present disclosure solve one or more of the problems set forth above.